An optical-modulation-type display panel is used for many electronic apparatuses such as electronic paper, besides thin information terminal apparatuses such as mobiles. This optical-modulation-type display panel displays an image, by blocking incident light or allowing the incident light to pass therethrough, for each pixel. A liquid-crystal display panel may be an example of the optical-modulation-type display panel. In recent years, an optical-modulation-type display panel of a non-liquid-crystal type has also been widely in practical use.
On a back face side of such an optical-modulation-type display panel, an illumination unit (a backlight) is provided, and light outputted from this backlight is incident on the display panel to display an image. An edge light system suitable for a reduction in thickness is often used for the backlight. In the edge light system, a light source is disposed on a side face of a light-guiding plate, and the backlight serves as a surface emitter, when light taken in by the light-guiding plate from the light source travels inside the light-guiding plate while being totally reflected.
As a way of extracting this light propagating inside the light-guiding plate, there has been proposed a method of providing a diffusion modulation device and a reflection device in this order on a back face side of the light-guiding plate (see, for example, Patent Literature 1). The diffusion modulation device is capable of electrically switching an action state for incident light between a diffusion state and a transmission state, and is, specifically, a polymer dispersed liquid crystal (PDLC). On the other hand, for the reflection device, for example, a device having high reflectance such as a white diffuse reflection sheet may be used. In the illumination unit having such a configuration, transmitted diffused light is outputted from the polymer dispersed liquid crystal in a diffusion state to a back face side. However, this transmitted diffused light is returned to a front face side at the diffuse reflection sheet and therefore, it is possible to utilize the light outputted from the light source (the light-guiding plate), as irradiation light to the display panel, efficiently.